The present invention relates to a high NA (numerical aperture) objective lens installed in an optical pick-up of an optical disc apparatus that is able to record/reproduce a plurality of kinds of optical discs whose cover layers have different thickness. Particularly, the present invention relates to the objective lens that has a diffractive lens structure formed on a surface of a refractive lens.
The optical disc includes an information layer on which digital information is recorded, and a transparent cover layer that covers the information layer. A laser beam from the optical pick-up is converged to form a beam spot on the information layer through the cover layer. The distance between the optical pick-up and the information layer varies depending upon the thickness of the cover layer.
Namely, the thicker the cover layer is, the farther the distance to the beam spot from the optical pick-up is. For example, since the cover layer of a compact disc (CD) or that of a CD-R has the thickness of 1.2 mm, and the thickness of the cover layer of a digital versatile disc (DVD) is 0.6 mm, the optical pick-up is required to move the beam spot away from the optical pick-up by 0.6 mm in the cover layer (0.4 mm in air) when the DVD is replaced with the CD or the CD-R.
Although a paraxial beam spot moves as the objective lens is moved, the change of the thickness of the cover layer changes spherical aberration. If the optical pick-up only moves the objective lens when the disc is replaced, wave front aberration of the laser beam is deteriorated, thereby the diameter of the beam spot is increased, which prevents the optical disc apparatus from reproducing the recorded information from the CD. For instance, when the objective lens, which is designed to minimize the spherical aberration when the recorded information is reproduced from the DVD, is used for reproducing the information from the CD, the spherical aberration becomes too large to reproducing the information even if the objective lens moves to bring the beam spot to be coincident with the information layer.
Therefore, there is known, as a prior art, an optical pick-up that adjusts the condition of the laser beam entering into the objective lens according to the thickness of the cover layer.
For example, Japanese Provisional Patent Publication No. Hei 7-98431 discloses such an optical pick-up. The optical system shown in this publication employs a holographic lens on the laser source side of the objective lens to divide the laser beam from the laser source into a zero order parallel diffractive beam and a first order divergent diffractive beam. The zero order diffractive beam is used for the optical disc having a thinner cover layer (i.e., the DVD), the first order diffractive beam is used for the optical disc having a thicker cover layer (i.e., the CD and CD-R). The optical pick-up of the publication enables to form the diffraction-limited beam spots for the respective optical discs when the holographic lens is designed to obtain the most suitable laser beams according to the thickness of the cover layers.
However, since the optical pick-up of the publication always divides the laser beam from the laser source into the zero and first order diffractive beams, and only one of these beams is used for recording/reproducing information at a time, the maximum efficiency in use of the light quantity is not more than 40%.
Further, when one of the diffractive beams is being used for recording/reproducing the information, the other diffractive beam is an unnecessary beam, which increases noises.
Still further, the recording density of the DVD is higher than that of the CD, which requires the optical pick-up for the DVD to form a smaller beam spot than the optical pick-up designed for the exclusive use of the CD (hereinafter referred as an exclusive CD pick-up). Since the diameter of the beam spot has a positive correlation with the wavelength of the laser beam, the optical pick-up for the DVD requires the laser source whose oscillation wavelength is 635 through 665 nm that is shorter than the oscillation wavelength of the exclusive CD pick-up (i.e., 780 through 830 nm). On the other hand, the reflection characteristics of the CD-R requires the laser source whose oscillation wavelength is about 780 nm.
Accordingly, when the optical pick-up having a single laser source as described in the publication employs a laser source that emits a laser beam having a shorter oscillation wavelength, it cannot reproduce the information from the CD-R.